Most animals and plants are sexual – they have a stage in their life-cycle at which genes from different individuals are recombined to produce genetically mixed progeny. Numerous evolutionary theories have been proposed for why sex is so prevalent, and these theories have important consequences for understanding many other fields, including speciation and molecular evolution. We study one notable and intriguing counter-example – the bdelloid rotifers – as a case study for gaining deeper understanding of the evolutionary causes and consequences of sex.

Our work has focused on whether asexual organisms diversify into species. It has been argued that ‘species’ are a feature of sexual organisms and that asexuals should not diversify into species. However, we showed that bdelloids have diversified into independently evolving species that experience divergent selection on feeding morphology in distinct habitats. The pattern of genetic diversity differs, however, from genetic diversity in monogonont rotifers (which have a sexual stage in their life-cycle). We have also found molecular evidence of inefficient natural selection in bdelloids, as predicted by evolutionary theory, and demonstrated the role of broad-scale dispersal in explaining bdelloid diversity patterns.

We are now seeking to understand how bdelloids interact with other organisms, such as parasites and symbionts, and the genetic mechanisms of adaptation to different environments. With Chiara Boschetti, Alan Tunnacliffe and colleagues at the University of Cambridge, we found that up to 10% of expressed genes in bdelloid rotifers come by horizontal transfer from bacteria, protists, plants and fungi. Current projects use whole genome sequencing to investigate the role of horizontal transfer in adaptation and speciation.

Scanning electron micrographs of species of Rotaria and their jaws (trophi, not drawn to scale) by Diego Fontaneto